class_docstr: Unit tests for the LangChain wrapper LLM class
code: def langchain_llm(self):
      class FakeOpenAI(OpenAI):
          openai_api_key = "fake_key"
  
          def __call__(
              self, _prompt, stop=None, callbacks=None, **kwargs
          ):
              return Mock(return_value="Custom response")()
  
      return FakeOpenAI()
  
file_path: pandas-ai/tests/llms/test_langchain_llm.py
imports: from langchain.llms import OpenAI
  import pytest
  from pandasai.llm import LangchainLLM
  from pandasai.prompts import AbstractPrompt
  from unittest.mock import Mock
label_desc: Use image node_id 1 for calling the TestLangchainLLM obj's underlying member method code with example usage: obj.langchain_llm() and returns: FakeOpenAI, Mock